Subject: Re: BUG: git grep behave oddly with alternatives

From: Junio C Hamano <[email-masked]>
Date: Wed, 04 Jan 2023 15:13:29 +0900

René Scharfe <[email-masked]> writes:

>    "For enhanced basic REs, ‘+’, ‘?’ and ‘|’ remain regular characters,
>     but ‘\+’, ‘\?’ and ‘\|’ have the same special meaning as the
>     unescaped characters do for extended REs, i.e., one or more
>     matches, zero or one matches and alteration, respectively."
>
> So apparently Apple chose a middle ground between basic and extended
> regular expressions for its grep and git grep.

Sounds like GNU extension to me.

> So GNU grep apparently made the same choice as Apple, probably far
> earlier.

Yup.

> Based on that I suggest:
> ...
>
> It would be simpler to use REG_ENHANCED everywhere it is defined instead
> of adding a Makefile setting, but it's a non-standard extension and
> might mean something else on other platforms.

OK.  Very conservative and good.

> Reported-by: Marco Nenciarini <[email-masked]>
> Signed-off-by: René Scharfe <[email-masked]>
> ---
>  Makefile         | 8 ++++++++
>  config.mak.uname | 1 +
>  grep.c           | 4 ++++
>  3 files changed, 13 insertions(+)
>
> diff --git a/Makefile b/Makefile
> index db447d0738..15e7edc9d2 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -289,6 +289,10 @@ include shared.mak
>  # Define NO_REGEX if your C library lacks regex support with REG_STARTEND
>  # feature.
>  #
> +# Define GIT_GREP_USES_REG_ENHANCED if your C library provides the flag
> +# REG_ENHANCED to enable enhanced basic regular expressions and you'd
> +# like to use it in git grep.
> +#
>  # Define HAVE_DEV_TTY if your system can open /dev/tty to interact with the
>  # user.
>  #
> @@ -2037,6 +2041,10 @@ endif
>  ifdef NO_REGEX
>  	COMPAT_CFLAGS += -Icompat/regex
>  	COMPAT_OBJS += compat/regex/regex.o
> +else
> +ifdef GIT_GREP_USES_REG_ENHANCED
> +	COMPAT_CFLAGS += -DGIT_GREP_USES_REG_ENHANCED
> +endif
>  endif
>  ifdef NATIVE_CRLF
>  	BASIC_CFLAGS += -DNATIVE_CRLF
> diff --git a/config.mak.uname b/config.mak.uname
> index d63629fe80..14c145ae42 100644
> --- a/config.mak.uname
> +++ b/config.mak.uname
> @@ -147,6 +147,7 @@ ifeq ($(uname_S),Darwin)
>  	FREAD_READS_DIRECTORIES = UnfortunatelyYes
>  	HAVE_NS_GET_EXECUTABLE_PATH = YesPlease
>  	CSPRNG_METHOD = arc4random
> +	GIT_GREP_USES_REG_ENHANCED = YesPlease
>
>  	# Workaround for `gettext` being keg-only and not even being linked via
>  	# `brew link --force gettext`, should be obsolete as of
> diff --git a/grep.c b/grep.c
> index 06eed69493..a8430daaba 100644
> --- a/grep.c
> +++ b/grep.c
> @@ -502,6 +502,10 @@ static void compile_regexp(struct grep_pat *p, struct grep_opt *opt)
>  		regflags |= REG_ICASE;
>  	if (opt->pattern_type_option == GREP_PATTERN_TYPE_ERE)
>  		regflags |= REG_EXTENDED;
> +#if defined(GIT_GREP_USES_REG_ENHANCED) && defined(REG_ENHANCED)
> +	else
> +		regflags |= REG_ENHANCED;
> +#endif
>  	err = regcomp(&p->regexp, p->pattern, regflags);
>  	if (err) {
>  		char errbuf[1024];
> --
> 2.39.0